Why is (SiO)₄ calculated to Be tetrahedral, whereas (CO) ₄ is square planar? A molecular orbital analysis

Qualitative molecular orbital (MO) theory predicts that square-planar tetrasilacyclobutanetetraone D_4h-(SiO)₄ should, like D_4h-(CO)₄, have a triplet ground state, and the results of the (U)CCSD(T)-F12b/cc-pVTZ-F12//(U)B3LYP/6-311+G(2df) calculations, reported here, confirm this expectation. Calculations at the same level of theory find that square-planar tetrasilacyclobutanetetrathione D_4h-(SiS) ₄ also has a triplet ground state. However, these ab initio calculations predict that (SiO)₄ and (SiS)₄ both have a singlet state of much lower energy, with a tetrahedral (T_d) equilibrium geometry and six, electron-deficient, Si-Si bonds. In contrast, the lowest singlet state of (CO)₄ and of (CS)₄ is calculated to prefer a D_4h to a T_d geometry. An analysis, based on the second-order Jahn-Teller effect, rationalizes the influence of the electronegativity difference between A and Y in (AY)₄ on the energy difference between a D_4h and T_d geometry. This analysis predicts that (BF)₄ and (BCl)₄, which are isoelectronic with, respectively, (CO)₄ and (CS)₄, should both prefer a T_d to a D_4h equilibrium geometry. These qualitative predictions have been confirmed by our calculations, and (BCl)₄ is known experimentally to have a T_d equilibrium geometry.